This invention relates to a method of providing a thermoplastics sheath or sleeve tightly fitting around a length of crush resistant tubing.
An application for which this method is particularly suited is in the provision of electrically insulating plastics sheathing around the tail pipe of a submarine cable optical fibre regenerator housing. Such a tail pipe provides a hydrostatic pressure resistant duct from a gland in the wall of the regenerator pressure resistant vessel to a tie-off position to which the submarine cable itself is attached. The duct must be capable of withstanding the hydrostatic pressures existing at the ocean bed without crushing the optical fibres it contains, it must also provide an electrical connection for powering the regenerator and it must be adequately mechanically protected and electrically insulated from sea water.
In a conventional electrical regime submarine cable the electrical analogue of the tail pipe is typically provided by a length of stranded copper conductor encased in a thick polythene sheath. The sheathing in this instance can satisfactorily be provided by conventional extrusion coating. Conventional extrusion coating can also be used for applying plastics sheathing to tubing which is to form the tail pipe of a submarine cable optical regenerator, but this is not the preferred process. The reason is at least in part because it is desired to subject the tubing to as little bending as possible prior to forming it into its final working configuration, whereas implicit in a conventional extrusion process is the production of long continuous lengths that require reeling and unreeling. A further consideration is that by working in discrete lengths it is possible to provide one end of the tube with a special configuration designed to reduce the tendency for the sheathing to extrude under hydrostatic pressure through a gland into which that end of the tubing is fitted.